For some reason on my forum, my admins and moderators are unable to delete or move threads for a reason unbeknowst to me and when we try to, we get an error message stating that the user has gone idle for too long and is unable to delete/move thread.

looks like your forum's current theme is not compatible with MyBB 1.8.x

upgrading guidance Wrote:Visit your Admin Control Panel, then go to Templates & Style > Templates > Find Updated Templates.
This will show you a list of all the templates that have changed during the upgrade.

You can either revert these templates to their default - meaning all the changes you've made to it will be removed - or you can see a Diff Report which will show you exactly what's changed. If you have a custom theme installed, it is probably best that you look at the Diff Report and apply the changes you need.